I just checked on latest develop and this is working as intended. The proficiency bonus(+2) is being applied on the following circumstances:

- you are only using mainhand and used an attack action

- you are using a light weapon on the offhand and used an attack action

- you are using a non-light weapon on the offhand but you have the ambidextrous trait and used an attack action

- you are unarmed or have a staff and are using a spell action

Valeria Boikova there is no penalty, only bonus to the attack roll.

I am checking if we are not applying the bonus to the mainhand on the attack roll like we're supposed to. That might indeed be a bug.

This still does not work, i had +2bonus on my offhand dagger today instead of -2.

edit: with mace equipped in below screen there is 0, should be -2

and on other with dagger eqipped there is 2 instead of -2.

We've fixed this. The offhand has no penalty. There is a mainhand bonus though.

also the +2 bonus for main hand is not applied